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help / color / mirror / Atom feedFrom: Andres Freund <[email protected]>
To: Alexander Korotkov <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: pg_atomic_compare_exchange_*() and memory barriers
Date: Sat, 8 Mar 2025 08:41:24 -0500
Message-ID: <vhimanxfy2h5hlfxeaoxqak4bfdovy35tbrd7o2jq6q5e27mc6@6ntlvt2n3ltk> (raw)
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Hi,
On 2025-03-08 08:02:41 -0500, Andres Freund wrote:
> From the C/C++ standard atomics model it doesn't make sense to say that a
> failed CAS has release semantics, as there simply isn't a write that could be
> ordered! What their barriers guarantee is ordering between multiple memory
> operation, you can't order multiple writes if you don't have multiple
> writes... The synchronization in the C/C++ model is only established between
> accesses of the same variable and there's no write in the case of a failed
> CAS, so there's nothing that could establish a release-acquire ordering.
>
> Unfortunately that model doesn't mesh well with barriers that aren't attached
> to read/modify operations. Which is what we ended up with...
Adding a full barrier to failed CAS would be a rather large overhead,
undesirable in just about any sane algorithm. As a consequence, I think what
we ought to do is to redefine the barrier semantics to only imply an acquire
barrier in case CAS fails.
Greetings,
Andres Freund
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To: [email protected]
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Subject: Re: pg_atomic_compare_exchange_*() and memory barriers
In-Reply-To: <vhimanxfy2h5hlfxeaoxqak4bfdovy35tbrd7o2jq6q5e27mc6@6ntlvt2n3ltk>
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